Selection — IF and CASE

Making a decision

• Selection means choosing what to do, based on a test.
• An if runs its block only when the test is true.
• The test gives a Boolean: True or False.

age = 20
if age >= 18:
    print("You may vote")

else: the other path

• else runs when the if test is false.
• Exactly one of the two blocks runs.

mark = 45
if mark >= 50:
    print("Pass")
else:
    print("Fail")

elif: many paths

• Use elif to test more conditions in order.
• Python checks each test top to bottom and stops at the first true one.

mark = 72
if mark >= 70:
    print("A")
elif mark >= 50:
    print("B")
else:
    print("C")

Comparison operators

• == means equal to (the exam writes =).
• != means not equal to (the exam writes <>).
• Also use <, >, <=, >=.

print(5 == 5)     # True
print(5 != 3)     # True
print(3 <= 2)     # False

In the exam: IF and CASE

• Pseudocode uses IF … THEN … ELSE … ENDIF, or CASE for many fixed choices.

IF Mark >= 50 THEN
    OUTPUT "Pass"
ELSE
    OUTPUT "Fail"
ENDIF

Common mistakes

• Compare with ==, not =.
• Cover every case, including the else.
• Indent the block that belongs to each branch.

Now you try

• Read the input, then choose what to print with if / elif / else.
• Press Check answer to test your code.